Of course when we send an ordinary email and think better of something, we might ignore trailing spaces etc. as unimportant - but when kmail notices them in a pgp-mime message, it knows they *must* be represented at the other end, and via 7 bit (rfc3156) too, so it uses qp(rfc2015).
A space, even when trailing, is still a 7-bit character, isn't it? I suspect KMail's misbehaviour is being triggered by a line length
72 but even that shouldn't trigger it. The trailing spaces seen
so far just looked like artefacts of other encoding-fu.